The ball screw comprises a screw shaft formed with a screw groove around the outer circumferential surface. A nut is formed with a screw groove on the inner circumferential surface. A plurality of balls are contained within a raceway formed by the opposite screw grooves of the screw shaft and the nut. The ball screw is used to convert a rotational force of the screw shaft (or the nut) to a thrust force of the nut (or the screw shaft) via the balls.
The ball, screw has a very high transmission efficiency due to the rolling contact of balls between the screw shaft and the nut. Thus, it is possible to convert the rotational motion to translational motion with a driving torque of about ⅓ that of the sliding screw. It is therefore possible to obtain a large thrust force by applying a small torque.
The CVT of the prior art is schematically shown in FIG. 5. The CVT comprises a pulley 50 on the input side and a pulley 51 on the output side, and a steel belt 52 wrapped around and extending between the pulleys 50 and 51. The input and output pulleys 50 and 51 are formed, respectively, by axially immovable pulley halves 50a and 51a and axially movable pulley halves 50b and 51b. The continuous speed variation can be achieved by axially shifting the axially movable pulley halves 50b and 51b and thus varying the belt wrapping radials of the input and output pulleys 50 and 51.
An actuator to shift the axially movable pulley halves 50b and 51b is formed by ball screws 53. As shown in FIG. 6(a), each ball screw 53 includes a screw shaft 54 and a nut 55 mounted thereon via a plurality of balls 58. The balls 58 are contained in a raceway formed by opposite screw grooves 56 and 57 so that they are infinitely circulated. These balls 58 are all load supporting balls having the same diameter as shown in FIG. 6(b).
The screw shaft 54 of the ball screw 53 is supported by a supporting member (not shown) formed integrally with a casing (not shown) so that the screw shaft 54 cannot be moved in both a rotational and axial direction. The nut 55 is supported movably in both a rotation and axial direction. Accordingly, the axially movable pulley halves 50b and 51b connected to the nuts 55 via bearings (not shown) can be translated along the screw shaft 54 by rotating the nuts 55 (see Japanese Patent Publication No. 33170/1996).
When an automobile provided with the CVT is running on a town street, the CVT is, in usual, frequently shifted in a narrow range between Lo-speed side and Hi-speed side. In this narrow range, the shifting range of the movable pulley halves 50b and 51b is very short.
Under the circumstances, the balls 58 suffer from friction and damage due to lack of lubrication in local regions especially between surfaces of adjacent balls 58 rotating in “counter” directions as shown by arrows in FIG. 6(b). This causes relative slippage between contacting points of adjacent balls. Thus, this lowers the mechanical efficiency of the ball screw 53 and diminishes the smooth speed change of an automobile.
In addition, since the nuts 55 have to be rotated in the CVT of the prior art to shift the axially movable pulley halves, ball circulating portions (not shown) formed in the nuts 55 are also rotated together with the nuts 55. Since a gap between the balls 58 and a raceway in the ball circulating portion is larger than that of the raceway formed by the screw grooves 56 and 57, the balls 58 cannot support the moment load and the radial load acting on the ball screw 53 when they are in the raceway in the ball circulating portion.
Accordingly it is necessary, in the CVT of the type of nut rotation, to increase the load supporting capacity or the rigidity of the ball screw 53 by enlarging the size of the balls in order to compensate for a deficiency of load supporting capacity. This diminishes a reduction of the weight and size of the CVT and makes a reasonable and fit design difficult.